The technology to thaw and heat frozen food such as frozen sushi by placing the food on a thawing-heating tray and irradiating it with microwave has been disclosed in JPA 8-180970 and JPA 9-98888. A tray claimed in claim 6 and disclosed in the second embodiment described in paragraphs 0019 through 0022 of JPA 8-180970 has a thick portion in the center thereof and is therefore most similar with the tray of the present invention. JPA 8-180970 will be hereinafter explained with reference to FIGS. 7 through 10.
FIG. 7 is a sectional view of a microwave thawing-heating apparatus for thawing sushi according to the first embodiment of the aforementioned JPA 8-180970. Microwave is supplied in a thawing-heating chamber 10 from magnetron through a wave guide 11. A rotating table (a disk) 12 made of conductive material such as aluminum is disposed in a lower part of the thawing-heating chamber 10. The rotating table 12 is rotated by driving means. Disposed on the rotating table 12 through a supporting member 13 made of material such as expanded polystyrene is a surface-wave-forming plate (a tray) 14 on which some pieces of sushi 3 are placed. The distance between the ceiling (upper surface) of the thawing-heating chamber 10 in which the wave guide 11 is arranged and rotating table 12 is set approximately n.lambda./2 (n is integer) wherein .lambda. is wave-length of microwave. In the thawing-heating chamber 10, an electric field of a standing wave F is formed vertically as illustrated. The aforementioned surface-wave-forming plate 14 is located close to a part where the electric field strength of standing wave F is high.
This surface-wave-forming plate 14 is made of dielectric material and functions to form surface wave (plane wave) and is held by the supporting member 13. As for this surface-wave-forming plate 14, among some dielectric materials, the material, the dielectric loss angle is less than 0.01 and the relative permittivity .epsilon..sub.0 is more than 40 is preferable, so that, alumina having relative permittivity .epsilon..sub.0 approximately 9 is the most suitable.
As shown in FIG. 8, microwave radiated from wreak guide 11 impinges into the surface-wave-forming plate 14 through the upper surface and the sides thereof. Moreover, the permeated microwave reflects by the rotating table 12 made of conductive material and thus impinges into the surface-wave-forming plate 14 through the bottom thereof. In this way, the microwave is transmitted within the surface-wave-forming plate 14 as indicated by arrows 100. At this time, on the surface of surface-wave-forming plate 14, surface wave of microwave is formed. Since the surface-wave-forming plate 14 is located on the part where the electric field strength of the standing wave F formed between the ceiling and the rotating table 12 in the thawing-heating chamber 10 is high, the strength distribution formed finally by this standing wave and the surface wave mentioned above is indicated as the numeral 200 in the drawing. Accordingly, the longer the distance from the surface-wave-forming plate 14, the smaller the electric field strength formed on the surface-wave-forming plate 14. According to the microwave electric field distribution, warm shari (pre-steamed rice) 1 and a cold neta (raw fish) 2 can be provided.
By the way, the surface-wave-forming plate 24 has the size such that frozen sushi 3 for four meals can be placed on the top. When the size of surface-wave forming plate 24 is larger, the formation of the surface wave in the central region becomes worse and it will be unable to thaw and heat all pieces of the frozen sushi 3 on the surface-wave-forming plate equally, because the strength distribution 401 in the central region becomes smaller than the strength distribution 400 in the peripheral region as shown in FIG. 9.
Therefore, the surface-wave-forming plate 24 which is thicker in a central portion 24C as shown in FIG. 10 is employed. By making the central portion 24C of the surface-wave-forming plate 24 thicker as mentioned, the central portion 24C functions as an adjuster, thereby making microwave propagation efficiently and also ensuring uniform electric field strength all over the surface-wave-forming plate 24, consequently.
In addition, according to JPA 8-180970, the surface-wave-forming plate 24 is made of alumina, relative permittivity of which is approximately 9, and is set to have a diameter of 500 mm so as to enable processing of, for example, 32 pieces of sushi for four meals at a time, and is set to have a thickness of 5 mm in the peripheral region thereof. The distance (the stratum of air) between the rim portion and the rotating table 22 is set to 17.5 mm by a support 23 made of synthetic resin or alumina. And also, the area within the diameter of 200 mm in the central portion 24C of the surface-wave-forming plate 24 has a thickness of 12.5 mm and the distance between the central portion 24C and the rotating table 22 is set to 10 mm.
Inventors of the present invention repeated experiments of the surface-wave-forming plate, the central portion 24C of which is thick as shown in FIG. 10, and found that, in case of the surface-wave-forming plate (hereinafter, sometimes called as "tray") made of alumina, dielectrics of which is large as mentioned above, the electric field strength gets "unevenness" or "irregularity" and it was therefore unable to thaw and heat the frozen sushi 3 every time in the same way.